We allow homebrew/dupes for instance, rather than Homebrew/dupes. Because nobody likes shifting in the terminal.
In the process of doing this I discovered some case-insensitive filesystem bugs we have avoided before because I had the foresight to mandate lowercase in formula names. GitHub considers Homebrew and homebrew to be different (even though you can't create both). So we had to allow case insensitivity in tap input. I have made it now so the resulting directory however is lowercased, neatly avoiding the issue. And so we also downcase tap arguments when applying them to tap directories or formula.